Interest in making well-defined linear polymers substituted with functional groups has been spurred, in part, by the commercial utility of ethylene-vinyl alcohol (EVOH) and poly(vinyl alcohol) (PVOH). EVOH and PVOH exhibit excellent barrier properties toward gases and hydrocarbons and have found use in the food packaging, biomedical, and pharmaceutical industries. See Lagaron et al. (2001) Polym. Testing 20:569–577, and Ramakrishnan (1991) Macromolecules 24:3753–3759. The polymers also provide extraordinary chemical and abrasion resistance. However, synthesis of these polyalcohols has required a circuitous and expensive route. Vinyl alcohol monomer tautomerizes to acetaldehyde, which precludes its use as a monomer.
As a result, the most widely employed synthetic route to EVOH is the free radical polymerization of ethylene and vinyl acetate to produce poly(ethylene vinyl acetate) (EVAc), which can then be converted to EVOH by saponifcation. See Mecking et al. (1998) J. Am. Chem. Soc. 120:888–899; and Ramakrishnan (1990) Macromolecules 23:4519–4524. These EVOH copolymers contain a degree of branching, much like low-density polyethylene (LDPE), and have a random distribution of alcohol functionality along the polymer backbone (Ramakrishnan (1991); Valenti et al. (1998) Macromolecules 31:2764–2773). PVOH is prepared in a similar manner by hydrolysis of poly(vinyl acetate), and analogous problems have been encountered. An additional drawback of the conventional processes for preparing these polymers, or other polyolefins substituted with functional groups, is that radical polymerization technology provides little means of controlling molecular weight (MW), molecular weight distributions (MWD), block structures, and incorporation of comonomers, and therefore enables poor control over the polymer's mechanical properties as well.
The direct incorporation of polar functional groups along the backbone of linear polymers has been made possible via ring-opening metathesis polymerization (“ROMP”) due to the development of functional group-tolerant late transition metal olefin metathesis catalysts. For example, Hillmyer et al. has reported the ROMP of alcohol-, ketone-, halogen-, and acetate-substituted cyclooctenes with a ruthenium olefin metathesis catalyst (Hillmyer et al. (1995) Macromolecules 28: 6311–6316). The asymmetry of the substituted cyclooctene, however, resulted in head-to-head (HH), head-to-tail (HT), and tail-to-tail (TT) coupling, yielding a polymer with regiorandom placement of the functional groups. A similar problem was encountered by Chung et al., who reported the ROMP of a borane-substituted cyclooctene with an early transition metal catalyst followed by oxidation to yield an alcohol functionalized linear polymer (Ramakrishnan et al. (1990), supra). A solution to this regiorandom distribution of functional groups was reported by Valenti et al., who used the acyclic diene metathesis (ADMET) polymerization of an alcohol-containing symmetric diene (Valenti et al., supra; Schellekens et al. (2000) J. Mol. Sci. Rev. Macromol. Chem. Phys. C40:167–192)) However, the molecular weights of these polymers were restricted to <3×104 g/mol by ADMET, and their rich hydrocarbon content has limited the barrier properties of the final EVOH copolymers (Lagaron et al., supra).
Single-site organometallic catalysts have been shown to provide exceptional control of MW, MWD, microstructure, and thus the mechanical properties of the polymers synthesized. Recently, a number of researchers have developed second-generation single-site catalysts using late metal complexes with specially substituted diimine ligands. See, e.g., Mecking et al. (1998), supra; Johnson et al. (1996) J. Am. Chem. Soc. 118:267–268; and International Patent Publication No. WO 96/23010. These catalysts have the important advantages of relatively low cost, ease of synthesis and support, and, in certain cases, tolerance of functional groups. These catalysts have been used to create a new class of very highly branched polyethylene, high-density polyethylene, atactic polypropylene, and a variety of polymers incorporating functional monomers. The catalysts do not, however, allow for polymerization of vinyl acetate and alkyl vinyl ethers such as butyl vinyl ether.
Accordingly, there is a need in the art for a method of synthesizing polymers using catalysts that are tolerant of functional groups and a process that enables precise control over molecular weight and molecular weight distribution. Ideally, such a method would also be useful in the synthesis of stereoregular polymers.